Not so easy way to know about your Kernel ABI

My work in Red Hat is related to the Linux kernel ABI (or kABI). This has been an entirely new and exciting experience for me since I became a Red Hat associate in May last year. In this blog post and some posts subsequent to this I would like to talk about everything I have been learning so far.

First up, ABI or the Application Binary Interface is a low level (binary level) interface exposed by some application. The linux kernel and its accompanying modules also expose (or exports) a binary interface which we refer to as the kernel ABI or kABI. It is important that this exported interface remains stable because modules written for a particular kABI would need to be recompiled if the interface does change in a subsequent kernel version.

The exported binary interface that I keep referring to is nothing except a list of symbols which have been decided to be exported. These symbols could be function names, enums, structs or anything. To find these exported symbols, you would just need to ‘grep’ through the linux source for “EXPORT_SYMBOL”. Following is a section of the grep output when I search for EXPORT_SYMBOL in the source code for kernel-3.14.5-200.fc20 built for Fedora:

sbairagy@dhcp193-120 ~/r/k/linux-3.14> grep -Rn "EXPORT_SYMBOL" .

All symbols enclosed in brackets after EXPORT_SYMBOL or EXPORT_SYMBOL_GPL are exported as a part of the binary interface exported by the kernel. When the kernel is built for Fedora, these exported symbols are stored in a file called Module.symvers. You can find this file for the kernel version you are running in /usr/src/kernels/<version>/. A section of the Module.symvers file for the kernel I am using is shown below:

sbairagy@dhcp193-120 ~/r/k/linux-3.14> vim /usr/src/kernels/3.14.8-200.fc20.x86_64/Module.symvers
0x00000000      tveeprom_read   drivers/media/common/tveeprom   EXPORT_SYMBOL
0x00000000      cx2341x_log_status      drivers/media/common/cx2341x    EXPORT_SYMBOL
0x00000000      ttm_bo_acc_size drivers/gpu/drm/ttm/ttm EXPORT_SYMBOL
0x00000000      inet_sendmsg    vmlinux EXPORT_SYMBOL
0x00000000      zs_get_total_size_bytes vmlinux EXPORT_SYMBOL_GPL

The 0th column contain checksums corresponding to the exported symbols which one can find in column 1. You might notice that the checksums for all the symbols are 0x00000000. This is not odd because some variable called CONFIG_MODVERSIONS had not been set during the kernel build process thus disabling generation of checksums for the exported symbols.

These checksums are vital to catching kABI breakages. kABI breakage occurs whenever changes are made to exported symbols. As a simple example, if we have a exported symbol ‘colours’ which refers to an enum ‘colours’ defined as

enum colours {"Red", "Green", "Blue"}

and if we decide to change this enum by adding one more colour (say “Yellow”) to it, then we say that the change in the exported symbol ‘colours’ have resulted in a change in the kernel ABI. Now whenever such changes happen for any symbol, the checksum corresponding to that symbol also change which leads to easy detection of kernel ABI change (or breakage).

How are these checksums generated? Well, there is a tool called genksyms which is integrated with the Linux kernel source and does this job of generating checksums for exported symbols during build process, if CONFIG_MODVERSIONS variable is set. In my subsequent posts I will write about the genksyms tool and how it actually generates the all important checksums. Till then, cheers. 😉


1 thought on “Not so easy way to know about your Kernel ABI

  1. Pingback: spartakus: Using sparse to have semantic checks for kernel ABI breakages | Samikshan Bairagya

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